Radio transmitter



Nov. 30, 1943. R. B. LINCOLN ET AL RADIO TRANSMITTER Filed April 11, 1941 Patented Nov. 30, 1943 UNITED STATES PATENT OFFICE RADIO TRANSMITTER ware Application April 11, 1941, Serial No. 388,140

11 Claims.

The present invention relates to radio transmitters and, more particularly, to such transmitters including a crystal oscillator driving a power amplifier.

An object of the present invention is the elimination of the necessity for separate power supplies for the oscillator and power amplifier in transmitters.

Another object is the provision of a crystal oscillator power amplifier transmitter having only a single tuning control.

Another object is the provision of a two stage transmitter having only a single tuning control.

A further object is the provision of a transmitter easily adaptable to break in operation.

A further object is the provision of a transmitter adaptable to break in operation which does not require the use of a fixed bias supply in the amplifier.

Still another object is the elimination of the necessity for a buffer amplifier between the oscillator and output amplifier stages of a moderate power transmitter.

Still a further object is the provision of a radio transmitter which is selectively operable as a continuous wave telegraph transmitter or as an efficient voice modulation transmitter having a linear modulation characteristic.

Still a further object is the provision of a transmitter, as aforesaid, which is inexpensive to construct and which has a low cost of operation.

The foregoing objects, and others which may appear from the following detailed description, are attained by the provision of a multi-grid radio frequency power amplifier in which the screen grid electrodes have current requirements of the same order of magnitude as required for the oscillator or other driver tubes. The screen voltage supply circuit has connected in series therewith the anode to cathode current path of the preceding oscillator or driveramplifier stages.

The present invention will be more fully understood by reference to the following detailed description which is accompanied by a drawing in which Figure 1 is a circuit diagram of an embodiment of the present invention and Figure 2 shows the equivalent direct current circuit of a portion of Figure 1.

Figure 1 shows a diagram of a transmitter constructed according to the present invention in which ID is the final or output amplifier tube. Tube H! is preferably a beam power tube having a high power handling capability and low driving power requirements. It may be of the type commonly known as the 813 transmitter beam power amplifier tube. It contains within its envelope an anode beam forming electrodes l2, screen grid I3, control grid I4 and cathode l5. The beam forming electrodes l2 are grounded as shown in the drawing. In series between anode I and the anode supply source indicated by the legend 2000V+, is connected the tuned tank output circuit l8 comprising inductance I9 and condenser 20. This tank circuit is the only tuned circuit in the entire transmitter which must be closely adjusted. Between the tank circuit and the anode supply source is connected a filter circuit comprising a radio frequency choke 9 and a shunt condenser 8. The cathode l5 of tube I0 is energized from an alternating current power source 2| through a transformer 22 having its secondary center tap grounded or connected to the negative side of the anode supply circuit. The filamentary cathode |5 is by-passed to ground at each end by condensers 23, 24 in a conventional manner. Control grid I4 is energized by radio frequency energy from the preceding driver or oscillator applied across grid leak 25. Screen electrode I3 is energized from the source of anode potential through a series circuit comprising resistor 26, choke 21 and anodecathode circuit of tube 30. Resistor 28 by-passed by condenser 29 completes the screen grid potential divider circuit across the anode power supply.

Tube 38 in the embodiment shown is a receiving type beam power tube such as the GVG-G having a plate current requirement approximately equivalent to the screen current requirement of tube It! and having sufficient power handling capabilities to fully drive the input of tube ID. The tube 33 is connected into a fixed tuned band switching crystal oscillator circuit. Anode 3| and control grid 34 are coupled to crystal 35 through condensers 36 and 31 and incandescent lamp 38 is connected in series with the crystal to indicate crystal current and also to act as a fuse. The grid-cathode circuit of tube 30 is completed by resistor 39 shunted by feedback condenser 40. Screen grid 33 is energized through a series resistor 4| and resistor 26 and by-passed to cathode 35 by condenser 42. Condenser 43 across the anode supply for tube 30 is a conventional high frequency by-pass condenser.

In case tube 30 has a lesser current handling the shell must be connected to cathode, as shown by the dotted line in Figure 2 and is, therefore, also at high potential with respect to ground. We, therefore, prefer to use a glass tube.

Due to the use of an untuned crystal oscillator for the driver of our transmitter, circuit adjustments for band and frequency shifting are reduced to a minimum. Tuning adjustments for the oscillator may be made once when the transmitter is constructed and thereafter left alone. There is also another reason for using an untuned oscillator in this circuitthat is to minimize the overloading of tube I in case of accidental outof-resonance operation of the oscillator circuit. If a tuned oscillator were used in this series circuit, accidental mistuning of the oscillator would lower the effective D.C. resistance of tube 33, therefore putting an excessive screen voltage on tube I0. This high screen voltage, plus the lack of grid bias, would result in excessive plate current in tube I El. A tapped untuned plate coil 21 is used in the oscillator so that high output can be obtained with low crystal current in any one of a plurality of predetermined frequency bands such as, for example, the 40, 80 and 150 meter amateur bands. The conventional form of untuned crystal oscillator with a pi-wound radio frequency choke for the plate circuit is quite satisfactory for 160 meter crystals. However, 40 and 80 meter crystals require critical adjustment of the feedback capacitance 40 to avoid excessive radio frequency crystal current. Also, any appreciable reduction of current through crystal 35 obtained by reducing the value of condenser 40 has the effect of lowering the output of the oscillator. These disadvanta s are largely overcome in the present invention by utilizing an untuned plate inductance 27 of the proper value for the lowest frequency band, for example, the 160 meter band and shorting out portions of the coil for the higher frequency bands such as the 80 and 10 meter bands by means of switch 45. When the optimum value for inductance 21 is used the oscillator output is a maximum, the crystal current is a minimum, and the value of condenser All is not critical. We have found that for the 160, 80 and 40 meter bands, coil 21 may consist of 155 turns of Number 28 enamel wire, close wound on a 1 /8 inch di-' ameter form. It should be tapped at 2I and 65 turns for the 40 and 80 meter bands, respectively. Different physical lay-outs of the transmitter, different coil-form sizes or wire sizes may require slight adjustment of the tap positions. If it is found that excessive crystal current occurs for any of the positions of tap switch 45 it is probable that the inductance of the portion of coil 21 included at that position is too large. If the crystal 35 is difiicult to start into oscillations the inductance of the particular portion of coil 21 included is probably too small. The anode of tube 30 is coupled to the grid leak 25 of tube III through a condenser 45.

In accordance with conventional practice closed circuit jacks 50 and are provided in the plate supply lead and grid return lead of tube I0 in order to measure the plate and grid currents. Jack 5| should, of course, be by-passed by a condenser 52 to avoid upsetting the operation of the transmitter when a meter is plugged into jack 5|.

For continuous wave telegraph operation the transmitter may be keyed by a keying relay 55 actuating contacts 5% in the screen supply circuit. The keying relay 55 may conveniently be energized from the secondary of transformer 22. A key click absorbing network comprising condenser 57 and series resistor 58 is connected across the contacts 56.

In Figure 2 we have shown a portion of the circuit of Figure l re-drawn in a somewhat different form in order to more clearly show how the poten tial for screen grid I3 is provided from a bleeder circuit across the anode potential supply source. The bleeder or potential divider comprises a series connection of resistor 28, coil 21, the anode-cathode path of tube 30 and resistor 28 shunted by the screen-cathode path of tube I 0. The potential relationships obtained in normal operation across this bleeder circuit are indicated on the drawing. All preliminary adjustments to the oscillator circuit of the transmitter should be made with the anode II of tube I0 disconnected and the supply voltage reduced to about 1000 volts either by using a different power supply or else by inserting a resistance of from 50,000 to 100,000 ohms in series with resistor 26 at the location indicated by an X in Figure 1. With these adjustments approximately 3 to 6 milliamperes of grid current in tube 30 may be obtained without lamp 38, which may conveniently have an operating current rating of 60 milliamperes, showing any color. The anode of tube I ll may then be reconnected and condenser 20 adjusted to tune circuit I8 to resonance for the desired frequency. Then a load may be coupled to coil I9 of tank circit I8 and the plate voltage supply returned to normal operating value. An ordinary 300-watt incandescent light bulb clipped across a portion of inductance I9 is a convenient output indicator and we have found that it is often much more satisfactory for testing purposes than an actual antenna load. With 2,000 volts on the anode of tube It and milliamperes plate current, the 300-watt bulb should light up to practically full brilliancy. After satisfactory output has been obtained the light bulb may be removed and the antenna connected.

It should be noted that when the tank circuit I8 is tuned to resonance the current drawn by screen I3 rises as resonance is approached. The increasing screen current increases the power input to the oscillator tube 30 because of the series arrangement. This, in turn, increases the output of oscillator 30, the current flowing through crystal 35 and the grid current of tube I0. These increases in grid and crystal currents are entirely normal and are not an indication of regeneration in the final amplifier tube Ill. We have found that it is best to keep the tank circuit I8 loaded at all times unless the anode voltage is reduced to approximately 1,000 volts, as mentioned earlier.

When filament voltage is applied to the tubes sufficient time should be allowed for the heater of tube 30 to reach operating temperature before applying high voltage. If this is not done excessive voltage may appear across the anode and cathode of tube 30 with resultant damage thereto.

The transmitter is keyed in the screen supply circuit because the relatively small amount of current that must be handled makes it relatively easy to eliminate key clicks. In addition, break in operation becomes possible without the use of fixed bias because the oscillator 30 is keyed simultaneously with the final amplifier ID. The filter comprising condenser 51 and resistor 58 across contacts 55 reduces key clicks considerably and also serves to extinguish any arcs across contacts 56. If the filter is omitted the spacing between contacts 56 in the open position must be nearly 4 inch to extinguish the are. We consider that it is absolutely necessary tame a keying relay in this circuit because of the high voltage which is handled.

In a transmitter actually built and tested for a considerable period the following values of circuit components were used:

Using the above values for telegraph operation, with 2,000 volts on the anode of tube 10, with 180 milliamperes plate current and 7 to 10 milliamperes grid current, a useful power output of 275 watts was obtained with an efliciency in excess of '75 percent, the total bleeder current or oscillator plate current being 25 milliamperes. For telephone operation, the transmitter was modulated by modulating the anode supply potential. The anode potential was reduced to Condenser Condenser Condenser 1,600 volts and resistor 28 omitted in order to insure correct screen voltage for tube H! with the reduced anode supply potential. Under these conditions the anode current of tube ID was 150 milliamperes with 7 milliamperes grid current. A useful carrier output of 175 watts was obtained with an efficiency in excess of 70 percent. Since the screen bleeder current was slightly less than 20 milliamperes the modulator must be able to completely modulate an input of 150+20 or 170 milliamperes at 1,600 volts. A conventional class B modulator capable of furnishing 135 watts of audio power was found to be sufficient to properly modulate the transmitter.

The following tests were made to check on the possibility of frequency modulation due to modulation of the oscillator anode supply in telephone operation:

1. Varying the applied plate voltage over a range of from 200 to 2,000 volts indicated no appreciable frequency shift aside from a slow drift due to change in temperature of the crystal.

2. Zero beating the carrier with another fixed carrier indicated no shift in average carrier frequency when modulation was applied.

3. There was no evidence of side band splatter of the type which generally results when frequency modulation is present.

It may, therefore, be concluded that frequency modulation in this transmitter is entirely negligible.

Further tests of the operation of this transmitter on several operating frequencies gave the following results:

Operating band 3. 5 3. 5 7.0 Resistor 28 000 Open 20, 000 Resistor 26 O, 000

Oscillator 30 Anode potential "volts. 315 230 200 Screen potentiaL. do 165 105 75 Grid potential do 25 20 15 Anode current. amperes 23 15 24 Screen current. do.. 3. 0 2.5 2. Grid current do.. 0. 5 0. 4 0.3

Amplifier Anode potential volts 2, 000 1,000 2, 000 Screen potential. ..do.. 340 400 400 Anode current. amperes 180 150 180 Screen current. "do. 7 17 10 Grid current do l0 7 7 While we have particularly shown and described several modifications of our invention, it

:90 mmfd. variable, 0.084 in. plate is to be distinctly understood that our invention is not limited thereto but that improvements withinthe scope of the invention may be made.

We claim:

1. In a radio transmitter, a power amplifier a driver tube coupled to said control electrode and having an anode-cathode current path therein, said anode-cathode path forming one part of said potential divider.

2. In combination, a first thermionic discharge tube having an anode, another electrode adapted to have positive potential applied thereto, a control electrode and a cathode, means for supplying positive potential from one terminal of a source to said anode, another terminal of said source being connected to said cathode, a potential divider connected across said source and having a tap connected to said other electrode, a second tube having an output coupled to said control electrode, said second tube having an anodecathode current path therein, said anodecathode path forming a part of said potential divider between the positive terminal of said source and said other electrode.

3. In a radio transmitter, a power amplifier tube having an anode, another electrode adapted to have positive potential applied thereto, a control electrode and a cathode, means for supplying a positive potential from one terminal of a source to said anode, another terminal of said source being connected to said cathode, a potential divider connected across said source and having a tap connected to said other electrode, a driver tube coupled to said control electrode and having an anode-cathode current path therein, said driver tube having current handling capabilities of the order of the current requirements of said source to said anode, another terminal of said source being connected to'said cathode, a potential divider connected across said source and having a tap connected to said other electrode, a driver tube coupled to said control electrode and having an anode-cathode current path therein, said anode-cathode path forming one part of said potential divider, and circuit controlling means between the positive terminal of said source and said other electrode for keying said transmitter.

5. In a radio transmitter, a power amplifier tube having an anode, another electrode adapted to have positive potential applied thereto, a control electrode and a cathode, means for supplying positive potential from one terminal of a source to said anode, another terminal of said source being connected to said cathode, a potential divider connected across said source and having a tap connected to said other electrode, a driver tube coupled to said control electrode and having an anode-cathode current path therein, said driver tube having current handling capabilities of the order of the current requirements of said other electrode, said anode-cathode path forming one part of said potential divider and circuit controlling means between the positive terminal of said source and said other electrode for keying said transmitter.

6. In a radio transmitter, .a power amplifier tube having an anode, another electrode adapted to have positive potential applied thereto, a control electrode and a cathode, means for supplying positive potential from one terminal of a source to said anode, another terminal of said source being connected to said cathode, a potential divider connected across said source and having a tap connected to said other electrode, a driver tube coupled to said control electrode and having an anode-cathode current path therein, said anode-cathode path forming a part of said potential divider between the positiveterminal of said source and said other electrode and circuit controlling means between the positive terminal of said source and said other electrode for keying said transmitter.

7. In a radio transmitter, a power amplifier tube having an anode, another electrode adapted to have positive potential applied thereto, a control electrode and a cathode, means for supplying a positive potential from one terminal of a source to said anode, another terminal of said source being connected to said cathode, a potential divider connected across said source and having a tap connected to said other electrode, a driver tube coupled to said control electrode and having an anode-cathode current path therein, said driver tube having current handling capabilities of the order of the current requirements of said other electrode, said anode-cathode path forming a part of said potential divider between the positive terminal of said source and said other electrode and circuit controlling means between the positive terminal of said source and said other electrode for keying said transmitter.

8. In a radio transmitter, a power amplifier tube having an anode, another electrode adapted lator tube, an untuned choke connected to said anode and to the positive terminal of said source, the anode of said oscillator being coupled to the control electrode of said amplifier tube, the cath-' ode of said oscillator being connected to said other electrode of said amplifier tube and means for keying said transmitter in series between said oscillator tube and said positive terminal of said source.

9. In a radio transmitter, a power amplifier tube having an anode, another electrode adapted to have positive potential applied thereto, a control electrode and a cathode, means for supplying positive potential from one terminal of a source to said anode, a tunable tank circuit in series between said anode and said source, another terminal of said source being connected to said cathode, an oscillator tube having an anode, a cathode and a control electrode, said oscillator tube having current requirements of the order of said other electrode of said poweramplifier, a crystal coupled to the anode and the control electrode of said oscillator tube, an untuned choke connected to said anode and to the positive terminal of said source, the anode of said oscillator being coupled to the control electrode of said amplifier tube and the cathode of said oscillator being connected to said other electrode of said amplifier tube.

10. In a radio transmitter, a power amplifier tube having an anode, another electrode adapted to have positive potential applied thereto, a control electrode and a cathode, means for supplying positive potential from one terminal of a source of modulated current to said anode, another terminal of said source being connected to said cathode, a potential divider connected across said source and having a tap connected to said other electrode, a driver tube coupled to said control electrode and having an anode-cathode current path therein, said anode-cathode path forming a part of said potential divider between the positive terminal of said source and said other electrode.

11. In a radio transmitter, a power amplifier tube having an anode, another electrode adapted to have positive potential applied thereto, a con-'- trol electrode and a cathode, means for supplying a positive potential from one terminal of a source of modulated current to said anode, another terminal of said source being connected to said cathode, a potential divider connected across said source and having a tap connected to said other electrode, a driver tube coupled to said control electrode and having an anode-cathode current path therein, said driver tube .having current handling capabilities of the order of the current requirements of said other electrode, said anodecathode path forming a part of said potential divider between the positive terminal of said source and said other electrode.

ROY B. LINCOLN. ANTHONY G. NEKUT. 

